Re: [PATCH 07/13] mm: close race in generic_access_phys

[Jason Gunthorpe]

On Wed, Oct 07, 2020 at 06:44:20PM +0200, Daniel Vetter wrote:
> Way back it was a reasonable assumptions that iomem mappings never
> change the pfn range they point at. But this has changed:
> 
> - gpu drivers dynamically manage their memory nowadays, invalidating
>   ptes with unmap_mapping_range when buffers get moved
> 
> - contiguous dma allocations have moved from dedicated carvetouts to
>   cma regions. This means if we miss the unmap the pfn might contain
>   pagecache or anon memory (well anything allocated with GFP_MOVEABLE)
> 
> - even /dev/mem now invalidates mappings when the kernel requests that
>   iomem region when CONFIG_IO_STRICT_DEVMEM is set, see 3234ac664a87
>   ("/dev/mem: Revoke mappings when a driver claims the region")
> 
> Accessing pfns obtained from ptes without holding all the locks is
> therefore no longer a good idea. Fix this.
> 
> Since ioremap might need to manipulate pagetables too we need to drop
> the pt lock and have a retry loop if we raced.
> 
> While at it, also add kerneldoc and improve the comment for the
> vma_ops->access function. It's for accessing, not for moving the
> memory from iomem to system memory, as the old comment seemed to
> suggest.
> 
> References: 28b2ee20c7cb ("access_process_vm device memory infrastructure")
> Cc: Jason Gunthorpe <jgg@xxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Benjamin Herrensmidt <benh@xxxxxxxxxxxxxxxxxxx>
> Cc: Dave Airlie <airlied@xxxxxxxx>
> Cc: Hugh Dickins <hugh@xxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-samsung-soc@xxxxxxxxxxxxxxx
> Cc: linux-media@xxxxxxxxxxxxxxx
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> ---
>  include/linux/mm.h |  3 ++-
>  mm/memory.c        | 44 ++++++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 44 insertions(+), 3 deletions(-)

This does seem to solve the race with revoke_devmem(), but it is really ugly.

It would be much nicer to wrap a rwsem around this access and the unmap.

Any place using it has a nice linear translation from vm_off to pfn,
so I don't think there is a such a good reason to use follow_pte in
the first place.

ie why not the helper be this:

 int generic_access_phys(unsigned long pfn, unsigned long pgprot,
      void *buf, size_t len, bool write)

Then something like dev/mem would compute pfn and obtain the lock:

dev_access(struct vm_area_struct *vma, unsigned long addr, void *buf, int len, int write)
{
     cpu_addr = vma->vm_pgoff*PAGE_SIZE + (addr - vma->vm_start));

     /* FIXME: Has to be over each page of len */
     if (!devmem_is_allowed_access(PHYS_PFN(cpu_addr/4096)))
           return -EPERM;

     down_read(&mem_sem);
     generic_access_phys(cpu_addr/4096, pgprot_val(vma->vm_page_prot),
                         buf, len, write);
     up_read(&mem_sem);
}

The other cases looked simpler because they don't revoke, here the
mmap_sem alone should be enough protection, they would just need to
provide the linear translation to pfn.

What do you think?

Jason